The copolymerization of ethylene and styrene by a conventional Ziegler-Natta catalyst is reported in Polymer Bulletin, 20, p. 237 (1988). Ziegler-Natta catalytic methods are commonly used throughout the polymer industry, especially for the production of ethylene copolymers, and have a long history tracing back to about 1957. However, for an vinyl aromatic comonomer such as styrene the polymerization activity for such Ziegler-Natta catalysts is low, such that an ethylene/styrene copolymer has a maximum of only about 1 mole percent styrene units in the copolymer. Furthermore, because of the heterogeneity of conventional Ziegler-Natta catalysts, the reported copolymer is actually a mixture of polymer chains of varying length, some having more than 1% by mole of styrene per individual chain and many, if not most, having no styrene groups.
Conventional Ziegler-Natta catalysts are actually composed of many types of catalytic species, each at different metal oxidation states and different coordination environments with ligands. Examples of such heterogeneous systems include metal halides activated by an organometallic co-catalyst such as, for example, titanium or magnesium chlorides complexed to trialkyl aluminum. Because these systems contain more than one catalytic species, they possess polymerization sites with different activities and varying abilities to incorporate comonomer into a polymer chain. The result of such multi-site chemistry is a product with poor control of the polymer chain architecture both within the sequence of a single chain, as well as when compared to a neighboring chain. In addition, differences in catalyst efficiency produce polymers of high molecular weight at some sites and low molecular weight at others.
Recently, a new catalyst technology useful in the polymerization of polyolefins has been introduced. Examples of introductory articles include "Exxon Cites `Breakthrough` in Olefins Polymerization," Modern Plastics, p.61, (July 1991); "Polyolefins Gain Higher Performance from New Catalyst Technologies," Modern Plastics, p.46, (October 1991); "PW Technology Watch," Plastics World, p.29, (November 1991); and Plastics Technology, p.15, (November 1991). These polymerization systems are based on the chemistry of metallocenes, which are organometallic compounds which contain one or more cyclopentadienyl ligands attached to metals such as hafnium, titanium, vanadium, or zirconium. A co-catalyst, such as, but not limited to, oligomeric methyl alumoxane, is often used to promote the catalytic activity of the system. By varying the metal component and the cyclopentadienyl ligand, a diversity of polymer products may be tailored having molecular weights ranging from about 200 to greater than 1,000,000 and molecular weight distributions from 1.5 to about 15. The choice of co-catalyst influences the efficiency and thus the production rate, yield and cost.
The uniqueness of metallocene catalysts resides in the steric and electronic equivalence of each catalyst position. Specifically, metallocenes are characterized as having a single, stable chemical type rather than a volatile mixture of states as discussed above for conventional Ziegler-Natta catalysts. This results in a system composed of catalyst positions which have a singular activity and selectivity. For this reason, metallocene catalyst systems are often referred to as "single site" owing to their homogeneous nature. Polymers and copolymers produced by such systems are often referred to as single site resins by their suppliers.
In recent years several resin suppliers have been researching and developing metallocene catalyst technology, with the polymers produced thereby having a narrow molecular weight distributions.
Olefin/vinyl aromatic copolymers have been prepared using metallocene catalysts. These polymers have only up to 50 mole percent of the aromatic vinyl polymerization units, because the active site of the catalyst becomes crowded with the incorporation. of the sterically hindered aromatic vinyl comonomer, making it unlikely, or impossible, that another hindered comonomer could enter into the polymerization as the next monomer in the sequence.
These olefin/vinyl aromatic copolymers are similar in at least some properties to other homogeneous, single site catalyzed copolymers such as the homogeneous, single site catalyzed ethylene alpha-olefins. That is, the present copolymers are characterized as having a narrow molecular weight distribution (MWD) and a narrow compositional distribution (CD). MWD refers to the breadth of the distribution of molecular weights of the polymer chains, and is a value which is obtained by dividing the number average molecular weight into the weight average molecular weight. The low CD, or regularity of side branch chains along a single chain and its parity in the distribution and length of all other chains, great reduces the low molecular weight and high molecular "tails". These features reduce extractables which arise from poor molecular weight control as well as improve optics by avoiding the formation of linear, ethylene-rich fractions.
However, at least in part because of the polarity of the vinyl aromatic comonomers used in the present invention, the present copolymers do not always follow the simplified trends of homogeneous, single site catalyzed ethylene alpha-olefins. Of course, it is the unique attributes of the present copolymers which make them desirable for use in a variety of film structures in accordance with the present invention. Yet, at least for those ethylene styrene copolymers which have a relatively low styrene content, generally from about 1% to about 10% by mole of styrene, processing and some physical properties are analogous to those of the homogeneous, single site catalyzed ethylene alpha-olefins. However, percent crystallinity drops much faster for the present copolymers than with any of the linear olefin comonomers of the homogeneous ethylene alpha-olefins. For the present copolymers 0% crystallinity occurs somewhere between 10% and 20% by mole of styrene. Thus, at least for those ethylene/styrene copolymers having between about 10% and 25% by mole of styrene, processing and some physical properties are analogous to known elastomers including the elastomeric homogeneous ethylene alpha-olefins. Yet, once again, a much greater percent by mole of alpha-olefin is required for homogeneous ethylene alpha-olefins to become elastomeric. Beyond 25% by mole of styrene such analogies to homogeneous ethylene alpha-olefins become less appropriate as the copolymer becomes more like polystyrene. Crystallinity generally remains at or about 0%; but, the glass transition temperature rises with an increase in amount of styrene sequences resulting in an increase in stiffness in resultant end products. However, the chemistry of the vinyl aromatic comonomer, whether it be styrene or a substituted styrene, is such that the present copolymers in each of the three comonomer content regions are beneficial when used in the production of appropriate film structures.